Goal: Reason regarding power in a circuit
Source: 283 circuit powers
Consider the circuit below. Which resistor has the greatest power
consumption?
Goal: Reason about electric circuits
Source: 283-580 100-watt and 40-watt bulbs in series
Two light bulbs are connected to a wall outlet as shown below. Bulb #1
is 100W and Bulb #2 is 40W. Which statement is true?
(3) Since the resistance of the bulbs goes as the reciprocal of its
wattage, students can reason that the most power is dropped in the 40W
bulb. Another way to reason is this. Suppose the potential source was
such that the potential difference over the 40W bulb was 120V, i.e. its
normal operating condition. The potential difference over the 100W bulb
is then 48V, which means that it is operating at (48/120)2 of its normal
condition. The bulb is dissipating about 16W.
Goal: Hone understanding of series circuits.
Source: 283-575 Two bulbs in series
A light bulb is connected to a battery as shown in Figure A below. When
a second bulb is connected as shown in Figure B, what happens to the
brightness of the original bulb?
(2) Depending upon the bulbs and battery, the bulbs could appear to go
out.
Goal: Hone understanding of parallel circuits.
Source: 283-570 lights in parallel
A light bulb is connected to a battery as shown in Figure A below. When
a second bulb is connected as shown in Figure B, what happens to the
brightness of the original bulb?
(3) If the battery cannot sustain the potential difference the bulb will
dim somewhat. This can confuse students so any demonstration should not
use a marginal battery.
Commentary:
Answer
(1) The potential drop over the 10Ω and 1000Ω resistors is the same.
Since power goes as V^2^/R, more power is consumed in the 10Ω resistor
than the 1000Ω resistor. Further, since power also goes as I^2^R and only
a fraction of the current through the 50Ω resistor flows through the 10Ω
resistor, the 50Ω resistor must dissipate the most energy.